Method of Manufacturing Silver Platelets

ABSTRACT

The invention provides an aqueous solution-based method for producing nanosized silver platelets, which employs the controlled mixing of a silver ion solution, a reducing solution, and an acidic solution in the presence of palladium ions.

This application claims priority to US Provisional Application Ser. No.60/742,031, filed Dec. 2, 2005, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of nanotechnology andnanoscale materials, and in particular to nanoscale silver platelets andmethods for their production.

BACKGROUND OF THE INVENTION

With their outstanding thermal and electrical conductivities, silverplatelets and silver flakes are an important class of materials, withapplications in many industries, such as the electronics industry, togenerate electrically conductive structures and to provide EMIshielding.

A variety of methods have been used to prepare silver flakes or silverplatelets, such as vertical freezing, ball milling, epitaxial growth,gas evaporation, vacuum deposition, Langmuir-Blodgett films, andchemical precipitation. The silver flakes used in the electronicindustry are almost exclusively produced by milling silver powders invarious solvents in the presence of suitable lubricants (See, e.g., U.S.Pat. No. 4,859,241 to Grundy). The flattening of the silver particlesresults from mechanical forces (shear and impact) provided by themovement of the milling media, which usually contains 1-5 mm spheres ofmaterials of different densities and compositions (glass, stainlesssteel, or ceramics). Because the majority of the silver powders used inthe milling process contain large agglomerates of sub-micrometer ormicrometer size particles, milling almost always leads to the formationof silver flakes with large average particle sizes (5-20 μm) and broadsize distributions. Such materials are becoming less and less suitablefor each succeeding generation of electronic devices, which requireincreasingly thinner and smaller conductive structures. Furthermore,friction- and impact-induced wear of the milling media leads tocontamination of the resulting silver flakes, thereby reducing theproduct quality.

Precipitation-based silver flake production is a promising technologyfor meeting the demands of the electronic industry. For example,nano-size silver flakes with edges up to about 110 nm have been producedby reducing silver nitrate with hydrazine at the interface of awater/octylarnine bi-layer system (Yener et al., Langmuir (2002)18:8692-99). However, such a system is not environmentally friendly, andwould be complex and costly on a commercial scale. Silver platelets inthe 30-120 nm size range were produced by irradiation of silvernanospheres (R. Jin et al., Nature (2003) 425:487-90), but this is atwo-step process; furthermore, photochemical processes are rarelyamenable to commercial-scale production.

There is a need in the art for silver flakes with widths in the range of0.1 to 1 μm that can be produced economically on a commercial scale.Consequently, there is great interest in the development of new,cost-effective, and environmentally friendly protocols that are capableof producing uniform silver flakes on a commercial scale.

SUMMARY OF THE INVENTION

The present invention provides methods for producing nanometer-scalesilver platelets, which include the essentially simultaneous addition ofa silver ion solution and a reducing solution to an acidic solutionunder conditions that permit the reduction of silver ions to metallicsilver particles, wherein the silver ion solution contains a pluralityof silver ions; the reducing solution contains one or more reducingagents; the acidic solution contains one or more stabilizing agents; andat least a part of the reduction of the silver ions and concurrentformation of the silver particles occurs in the presence of a pluralityof palladium ions.

In one embodiment, the reducing agent is either ascorbic acid orisoascorbic acid. In another embodiment, the acidic solution containsgum arabic. In yet another embodiment, the silver ion solution containsa plurality of palladium ions, and in still another embodiment, thereducing solution further contains gum arabic as a stabilizing agent.

Also provided are silver platelets produced according to the methods ofthe present invention, and compositions including such silver platelets.

The present invention further provides a metallic nano-plateletcontaining at least about 90% silver and less than about 10% palladium,and compositions including the same.

Other features and advantages of the present invention will becomeapparent from the following detailed description. It should beunderstood, however, that the detailed description and the specificexamples, while indicating the preferred embodiments of the invention,are given by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows FE-SEM images of silver platelets produced as described inExample 1. The images were obtained using a JEOL JSM-7400F system at 15kV accelerating voltage. Magnifications: (a) 10,000×; (b) 25,000×; (c)100,000×; and (d) 100,000×.

FIG. 2 illustrates the particle size distribution (PSD) of silverplatelets produced as described in Example 1. The PSD was determined bylaser diffraction using a Malvern 2000 size analyzer.

FIG. 3 shows an FE-SEM image at a magnification of 25,000× of silverplatelets produced as described in Example 2.

FIG. 4 shows an FE-SEM image at a magnification of 25,000× of silverparticles produced as described in Example 3.

FIG. 5 shows an FE-SEM image at a magnification of 25,000× of silverplatelets produced as described in Example 5.

FIG. 6 shows an FE-SEM image at a magnification of 25,000× of silverplatelets produced as described in Example 6.

FIG. 7 shows an FE-SEM image at a magnification of 50,000× of silverplatelets produced as described in Example 7.

FIG. 8 shows an FE-SEM image at a magnification of 50,000× of silverplatelets produced as described in Example 8.

FIG. 9 shows an FE-SEM image at a magnification of 50,000× of silverplatelets produced as described in Example 9.

FIG. 10 shows an FE-SEM image at a magnification of 100,000× of silverplatelets produced as described in Example 10.

FIG. 11 shows the X-ray diffraction (XRD) pattern of silver plateletsproduced in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein and in the appended claims, the singular forms “a,” “an,”and “the” include plural references, unless the content clearly dictatesotherwise. Thus, for example, reference to “a silver platelet” includesa plurality of such platelets and equivalents thereof known to thoseskilled in the art, and reference to “a reducing agent” is a referenceto one or more reducing agents and equivalents thereof known to thoseskilled in the art, and so forth. All publications, patent applications,patents, and other references mentioned herein are incorporated byreference in their entirety.

The term “nano-platelet” as used herein refers to particles having athickness of about 10 nm to about 500 nm, an average diameter or widthof about 100 nm to about 1000 nm, and an aspect ratio (width/thickness)ranging from about 2 to about 50.

The present invention provides a simple, cost-effective, andenvironmentally-friendly chemical method for producing smaller, thinner,and more uniform silver particles than have previously been available.More specifically, the method according to certain embodiments of theinvention produces flat metallic particles having a thickness of about20 nm to about 300 nm, a width of about 100 nm to about 1000 nm, and anaspect ratio (width/thickness) ranging from about 2 to about 20. In someembodiments, a majority of the nano-platelets are characterized by anaxis of symmetry, which is typically (but not limited to) 3-foldsymmetry.

The inventors have previously demonstrated the preparation of uniform,isometric nano-sized silver particles in acidic solutions, by thereduction of silver ions with ascorbic acid in the presence of variousdispersants (US Publication. No. 2006/0090598 to Goia et al., which isincorporated by reference in its entirety). The inventors have nowdevised a method for producing uniform, nano-sized, anisotropic silverparticles, i.e., silver nano-platelets, by manipulating the conditionsfor nucleation and early stages of particle growth in an acidic reducingsystem.

In at least one embodiment, the present invention provides a systemwhich produces silver platelets, and also the silver platelets producedthereby, that includes essentially simultaneous addition of a silver ionsolution and a reducing solution to an acidic solution under conditionsthat permit the reduction of a silver ion to metallic silver, therebyproducing the silver platelets.

As used herein, the expression “essentially simultaneous” refers toaddition of the silver ion solution and the reducing solution to theacid solution under one or more of the following conditions: (1) thetime period during which addition of one solution is carried outoverlaps a majority of the time period during which the other solutionis carried out; (2) the starting times for adding the two solutions donot differ by more than 10% of the total time for complete addition ofthe solutions; and (3) the completion times for addition of the twosolutions do not differ by more than 10% of the total time for completeaddition of the solutions.

Unlike the binary system known in the art by Yener et al., where silverplatelets are formed at the lamellar bilayer interface of a hydrophobicorganic compound-water system, the system of the present invention is asingle phase system in which silver platelets are formed in andprecipitated out of an aqueous solution.

The silver ion solution contains a plurality of silver ions, and canoptionally contain palladium ions. For example, a silver ion solutionmay contain about 0.94 M Ag⁺ and about 0.02 M Pd²⁺. In another example,a silver ion solution may contain about 0.99 M Ag⁺. The silver ionsolution can be prepared using any silver compound(s) known in the artwhich releases silver ions into solution to be reduced by a reducingagent to form metallic silver particles. In one preferred embodiment,the silver compound is silver nitrate.

The silver ion solution can be a homogenous solution, a gradientsolution, or a combination of solutions continuously or substantiallycontinuously applied during the process of the present invention. Agradient solution contains at least one non-uniformly-distributedsubstance. The gradient can be of any type, for example a lineargradient, a non-linear gradient, or a step gradient. Thenon-uniformly-distributed substance can be a plurality of palladiumions. The inventors have discovered that if, during the nucleation andthe early stages of particle growth, nitric acid (0.1-4.5 mol.dm⁻³) isprovided and Pd²⁺ ions are present, (introduced, for example, in thesilver nitrate solution at 0.5-10% based on the weight of silver), thegrowth mechanism of the metallic particles changes from isotropic growthto anisotropic growth, and highly dispersed silver nano-platelets areeventually formed. In one embodiment, the silver ion solution comprisesa preceding portion and a succeeding portion, wherein the precedingportion of the silver ion solution contains a plurality of silver andpalladium ions. The preceding portion and the succeeding portion can bepart of one solution, or be different solutions separately prepared andapplied during the process of the present invention.

The reducing solution of the present invention contains a reducingagent. The term “reducing composition,” or “reducing agent,” as usedherein, generally includes any water soluble reducing substance, andcombinations thereof, which is capable of reducing silver ions tometallic silver. Suitable reducing agents include, without limitation,acids, aldehydes, aldoses, monohydroxylic and polyhydroxylic alcohols(polyols), hydrazine, various boron and aluminum hydride species, andreducing saccharides (including monosaccharides, oligosaccharides, andpolysaccharides), as well as, where applicable, their sodium, potassium,and ammonium salts. In preferred embodiments, the reducing agent isascorbic acid or isoascorbic acid. For example, a suitable reducingsolution of the present invention is a 20% w/v solution of ascorbic acidin water.

The reaction mixture of the present invention can contain a stabilizingagent. The stabilizing agent can be present in one or more of the silverion solution, the reducing solution, and the acidic solution. In oneembodiment, both the reducing solution and the acidic solution contain astabilizing agent.

The term “stabilizing composition,” or “stabilizing agent,” as usedherein, generally includes any water-soluble stabilizing substance whichis capable of dispersing and stabilizing the newly formed silvernano-platelets in the reaction mixture, thus preventing undesirableaggregation of these particles. Suitable stabilizing agents are known inthe art, and include, without limitation, water soluble resins(including, e.g., naturally occurring, synthetic, and semi-syntheticwater soluble resins), gum arabic, water soluble polymers, water solublepolysaccharides, water soluble glycoproteins, various water solublesalts of naphthalene sulfonic-formaldehyde co-polymers, and combinationsthereof. In preferred embodiments of the present invention, thestabilizing agent is gum arabic.

In certain embodiments, the amount of halide in the raw materials iscontrolled so as to be as low as possible, because the formation ofnano-platelets is favored at low levels of halides. Keeping the level ofhalides, for example, chloride, at or below 20 ppm on a silver metalbasis promotes the formation of nano-platelets over spherical particles.

The stabilizing agent can be removed after the formation of thenano-platelets is complete. A number of protocols for removing thestabilizing agent are known in the art, such as, acid, alkaline, and/orenzymatic hydrolysis. In one embodiment, gum arabic is removed from thereaction mixture after the reaction through alkaline hydrolysis. Forexample, the hydrolysis of gum arabic may be performed for an extendedtime at high temperature (e.g., between about 60° and about 100° C., orbetween about 70° and about 90° C., or between about 78° and about 88°C.) and at high pH (e.g., pH 11.5). It is generally desirable tomaintain the pH of the mixture during the hydrolysis between about 8 andabout 14, preferably between about 10 and about 13, and more preferablybetween about 10.5 and about 12. The duration of the hydrolysis candepend upon a number of factors, such as temperature, pH, and the amountof stabilizing agent (e.g. gum arabic) present. For example, thehydrolysis of gum arabic can generally be performed for about 0.2 to 10hours, or about 0.5 to 5 hours, or about 1 to 3 hours.

The acidic solution of the present invention contains an acid, andoptionally a stabilizing agent. For example, the acidic solution can beprepared by dissolving 2.3 g of gum arabic in 320 ml H₂O, followed bythe addition of 20 ml of concentrated HNO₃. In certain embodiments, theconcentration of the nitric acid is from about 0.1 to about 4.5 M.

The resulting silver platelets can be isolated following standardprotocols known in the art, such as by precipitation, filtration, andcentrifugation. In one embodiment, centrifugation facilitates theproduction of silver platelets with a smaller size. For example, thereaction is carried out in a continuous centrifuge under conditionswherein the silver platelets with a smaller size precipitate withoutcausing substantial aggregation of the nano-platelets. The particles canthen be washed, for example by using methanol or ethanol, and dried,such as by air, N₂, or vacuum.

In one embodiment, the silver platelets contain about 90% or more silverand 10% or less palladium. In another embodiment, the silver plateletscontain about 99.1% or more silver, and about 0.8% or less palladium. Instill another embodiment, the silver platelets contain about 0.2%palladium. The incorporation of a trace amount of palladium into thesilver platelet of the present invention can significantly reduce themigration problems which are known to affect all silver-based particles,flakes, and platelets.

The silver platelets produced in accordance with the present inventionhave a thickness of about 10 nm to about 300 nm, or about 40 to about200 nm. In one embodiment, the silver platelets have an averagethickness of about 60 to about 100 nm.

The silver platelets produced in accordance with the present inventionhave a width of about 100 nm to about 1000 nm. In one embodiment, thesilver platelets have an average width of about 500 nm to about 1000 nm.The silver platelets of the present invention can be highly uniform insize.

In one embodiment, the method of the present invention producescompositions of silver platelets that have a tight size distribution forthe width and/or thickness (See Table 1). The breadth of a sizedistribution, as used herein, refers to the degree of variation in thedimension of interest of the silver platelets in a composition. Inpreferred embodiments, at least about 90% of the silver nano-plateletshave a width within the range of W±25% W, where W is the mean width ofthe silver platelets. In further preferred embodiments, at least about80% of the silver nano-platelets have a thickness within the range ofT±20% T, where T is the mean thickness of the silver platelets. Thethickness and width of the silver platelets can be measured by a numberof techniques, such as, by electron microscopy, particularly, scanningelectron microscopy (e.g., using a field emission scanning microscope).

The uniformity of the silver platelets produced in accordance thepresent invention can also be characterized by the aspect ratio(width/thickness) of the nano-platelets. The aspect ratio of the silverplatelets can range from about 2 to about 20.

In addition, in one embodiment, the silver platelets of the presentinvention can be highly crystalline. The term “degree of crystallinity,”as used herein, refers to the ratio between the size of the constituentcrystallites and the thickness of the nano-platelets. The size of theconstituent crystallites can be deduced from X-ray diffraction (XRD)measurements using Sherrer's equation, while the thickness of thenano-platelets can be determined by electron microscopy. A larger ratioof the size of the constituent crystallites in comparison to thethickness of the silver platelets indicates an increased degree ofcrystallinity. In one embodiment, the silver nano-platelets have a highdegree of crystallinity if at least about 80%, preferably, at leastabout 85%, more preferably, at least about 90-95%, and even morepreferably, about 100% of the silver nano-platelets are highlycrystalline.

The silver nano-platelets of the present invention can be substituted,wholly or in part, for prior art silver flake and silver powdercompositions in many applications. For example, conductive inks,coatings, and adhesives can comprise silver nano-platelets according tothe invention, together with thermoplastic and/or thermosettingpolymers, solvents, and various additives such as binders, stabilizers,Theological modifiers, and surfactants. Solvents, polymers, andadditives suitable for use in conductive inks, coatings, and adhesivesare well-known in the art; see for example U.S. Pat. No. 6,379,745(which is incorporated herein by reference) and references citedtherein. Another potential application for the silver nano-platelets isas an antimicrobial agent, in view of the well-known antimicrobial andantifungal properties of silver nano-particles.

EXAMPLES

The following examples illustrate specific embodiments of the presentinvention. They are set forth to aid in the understanding of theinvention, and should not be construed to limit in any way the scope ofthe invention as defined in the claims.

Materials: Silver nitrate (AgNO₃) was obtained from Ames Goldsmith Corp.(Glens Falls, N.Y.). Palladium nitrate (Pd(NO₃)₂) was obtained fromUmicore (Belgium). Gum arabic was obtained from Frutarom Incorporated(North Bergen, N.J.). Nitric and hydrochloric acids were obtained fromFisher Scientific Co. (Fair Lawn, N.J.). Ascorbic Acid was obtained fromRoche Vitamins Inc. (Parsippany, N.Y.). Deionized (“DI”) water was usedthroughout.

Example 1 Preparation of Silver Platelets (A) Precipitation Process

The silver nano-platelets were prepared by the double-jet addition ofsilver nitrate (AgNO₃) and ascorbic acid solutions into a highly acidicsolution of gum arabic in water. To generate the metallic particles asvery small nano-platelets, the total volume of silver nitrate solution(160 ml) was divided into two distinct parts (A₁ and A₂). The first partof the solution (A₁) was prepared by dissolving 1.91 g AgNO₃ in 11.14 mlH₂O and then adding 0.275 g Pd(NO₃)₂ solution (9.0% Pd metal) to providea concentration of 0.94 M Ag and 0.02 M Pd (final volume: 12 ml). Thesecond part (A₂) was prepared by dissolving 23.24 g AgNO₃ in H₂O (finalvolume: 148 ml), to provide an AgNO₃ solution with a concentration of0.99 M Ag. The reducing solution was prepared by dissolving 32 gascorbic acid and 1.5 g gum arabic in 160 ml H₂O. The acidic solutionwas prepared by dissolving 2.3 g gum arabic in 320 ml H₂O, followed bythe addition of 20 ml of concentrated HNO₃.

The precipitation was carried out by the addition, in parallel, of themetallic precursor solutions (first the A₁ solution followed immediatelyby the A₂ solution) and the ascorbic acid solution, over 90 minutes,into the 320 ml of acidified gum arabic solution, at room temperatureand with moderate agitation.

The time for introducing the reducing solution (containing ascorbicacid) and the total time for introducing the solutions containing silverion, did not differ by more than 3 minutes.

(B) Hydrolysis of Gum Arabic

The excess of gum arabic was removed by increasing the pH of the silverplatelet dispersion to 11.5 with 10.0 N sodium hydroxide and heating themixture to about 85° C., for 1 hour.

(C) Processing the Silver Platelets

When the hydrolysis of the gum arabic was complete, the dispersion wasallowed to cool and the silver platelets were allowed to settle. Thesupernatant was discarded, and the silver platelets were washed twicewith DI water. A third wash was carried out with 50% ethanol in DIwater, and two more washes with pure alcohol were performed. Theplatelets were dried overnight on filter paper at room temperature.

(D) Platelet Characterization

The morphology and size of the silver nano-platelets produced wereanalyzed by field emission scanning electron microscopy (FE-SEM) using aJEOL JSM-7400F device at 15 kV accelerating voltage and a magnificationbetween 10,000× and 650,000×.

FIG. 1 shows the FE-SEM images of the silver platelets obtained by theprocess described above. A large majority of the metallic particles werecrystalline silver platelets, with an average thickness of about 60 nmand an average width of 0.725 μm, for an average aspect ratio(width/thickness) of 12.

The particle size distribution was further confirmed by the laserdiffraction technique using a Malvern 2000 size analyzer (FIG. 2), whichindicated an average particle size of about 0.8 μm.

FIG. 11 shows the X-ray diffraction (XRD) spectrum for silver plateletsobtained by the process described above. The size of the constituentcrystallites was deduced using Sherrer's formula for the (111) peak. Bythis method the constituent crystallite size was calculated to be 56 nm.

Example 2

Using the method as outlined in Example 1: the first part of thesolution (A₁) was prepared by dissolving 19.2 g AgNO₃ in 112 ml H₂O andthen adding 1.608 g Pd(NO₃)₂ solution (15.0% Pd metal) providing aconcentration of 0.94 M Ag and 0.02 M Pd (final volume: 120 ml). Thesecond part (A₂) was prepared by dissolving 232.6 g AgNO₃ in H₂O (finalvolume: 1330 ml), to provide an AgNO₃ solution. The reducing solutionwas prepared by dissolving 320 g ascorbic acid (Alfa Aesar, chloridelevel based on ascorbic acid of less than 10 ppm, equating to a level ofless than 20 ppm based on silver) and 15 g gum arabic in 1600 ml H₂O.The acidic solution was prepared by dissolving 23 g gum arabic in 3200ml H₂O, followed by the addition of 200 ml of concentrated HNO₃. Theprecipitation was carried out by the addition, in parallel, of themetallic precursor solutions (first the A₁ solution followed immediatelyby the A₂ solution) and the ascorbic acid solution, over 90 minutes,into the 3200 ml of acidified gum arabic solution, at room temperatureand with moderate agitation. After conducting steps B, C, and D, thematerial was found to consist of silver platelets very similar to thosederived in example 1. FIG. 3 shows an FE-SEM image of the silverplatelets obtained in this example.

Comparative Example 3

The procedure as outlined in example 2 was followed except thatisoascorbic acid obtained from Sigma Aldrich was used in place of theascorbic acid. The chloride level of the isoascorbic acid was determinedto be 340 ppm, or 681 ppm based on the silver contained. Uponcharacterization, the material was found to consist of spherical silverparticles with no nano-platelet formation. FIG. 4 shows an FE-SEM imageof the silver particles obtained in this example.

Comparative Example 4

The procedure as outlined in example 2 was followed except that 0.4 mlconcentrated hydrochloric acid (37%) was additionally added to the acidsolution. The chloride level added via the hydrochloric acid wascalculated to be 1064 ppm based on the silver contained. Uponcharacterization, the material was found to consist of spherical silverparticles with no nano-platelet formation.

Example 5

Using the same method as outlined in Example 1, the first part of thesolution (A₁) was prepared by dissolving 33.4 g AgNO₃ in 195 ml H₂O andthen adding 2.824 g Pd(NO₃)₂ solution (15.0% Pd metal) to provide aconcentration of 0.94 M Ag and 0.02 M Pd (final volume: 120 ml). Thesecond part (A₂) was prepared by dissolving 407.1 g AgNO₃ in H₂O (finalvolume: 2330 ml), to provide an AgNO₃ solution. The reducing solutionwas prepared by dissolving 560 g ascorbic acid (Alfa Aesar, chloridelevel based on ascorbic acid of less than 10 ppm, equating to a level ofless than 20 ppm based on silver) and 26.3 g gum arabic in 1600 ml H₂O.The acidic solution was prepared by dissolving 40.3 g gum arabic in 5600ml H₂O, followed by the addition of 350 ml of concentrated HNO₃. Theprecipitation was carried out by the addition, in parallel, of themetallic precursor solutions (first the A₁ solution followed immediatelyby the A₂ solution) and the ascorbic acid solution, over 90 minutes,into the 5600 ml of acidified gum arabic solution, at room temperatureand with moderate agitation. After conducting steps B, C, and D, thematerial was found to consist of silver platelets very similar to thosederived in example 1. FIG. 5 shows an FE-SEM image of the silverplatelets obtained in this example.

The parameters for the batch produced in this example and the batchproduced in Example 2 are summarized in Table 1.

TABLE 1 Example 2 Example 5 Batch Size (g) 153 276 Width Parameters Avg.Width (nm) 622.6 646.8 Width St. Dev. (nm) 78.6 108.7 Minimum Width (nm)548.1 503.0 Maximum Width (nm) 821.0 975.2 Width within 10% of mean 60%60% Width within 20% of mean 92% 72% Width within 25% of mean N/D 96%Thickness Parameters Avg. Thickness (nm) 81.6 64.0 Thickness St. Dev.(nm) 10.3 10.4 Minimum Thickness (nm) 66.7 42.6 Maximum Thickness (nm)104.8 90.4 Thickness within 10% of mean 57% 44% Thickness within 20% ofmean 93% 88%

Example 6 99% Ag 1% Pd Platelets

Using the procedure as outlined in Example 1 except only one metallicprecursor solution was prepared: the metallic precursor solution wasprepared by dissolving 24.96 g AgNO₃ in 154 ml H₂O and then adding 1.78g Pd(NO₃)₂ solution (9.0% Pd metal) to yield a concentration of 0.95 MAg and 0.0097 M Pd. After conducting steps B, C, and D, the material wasfound to consist of nano-platelets approximately 770 nm wide×190 nmthick. FIG. 6 shows an FE-SEM image of the silver platelets obtained inthis example.

Example 7 98% Ag 2% Pd Platelets

Using the procedure as outlined in Example 1 except only one metallicprecursor solution was prepared: the metallic precursor solution wasprepared by dissolving 2.88 g AgNO₃ in 16.8 ml H₂O and then adding 0.415g Pd(NO₃)₂ solution (9.0% Pd metal basis) to provide a concentration of0.97 M Ag and 0.02 M Pd. The metal solution was added at a rate of 1.68ml/min for 10.5 minutes. After conducting steps B, C, and D, thematerial was found to consist of nano-platelets approximately 350 nmwide×36 nm thick. FIG. 7 shows an FE-SEM image of the silver plateletsobtained in this example.

Example 8 98% Ag 2% Pd Platelets

Using the procedure as outlined in Example 1 except only one metallicprecursor solution was prepared: the metallic precursor solution wasprepared by dissolving 1.91 g AgNO₃ in 11.14 ml H₂O and then adding0.275 g Pd(NO₃)₂ solution (9.0% Pd metal basis) to provide aconcentration of 0.99 M Ag and 0.021 M Pd. The acidic solution wasprepared by dissolving 2.3 g gum arabic in 320 ml H₂O, followed by theaddition of 60 ml of concentrated HNO₃, three times as much as added inExample 1. The metal solution was added at a rate of 1.68 ml/min for 7minutes. After conducting steps B, C, and D, the material was found toconsist of nano-platelets approximately 270 nm wide×40 nm thick. FIG. 8shows an FE-SEM image of the silver platelets obtained in this example.

Example 9 95% Ag 5% Pd Platelets

Using the method as outlined in Example 1 except only one metallicprecursor solution was prepared: the metallic precursor solution wasprepared by dissolving 23.93 g AgNO₃ in 144 ml H₂O and then adding 8.89g Pd(NO₃)₂ solution (9.0% Pd metal basis) to provide a concentration of0.94 M Ag and 0.05 M Pd. The metal solution was added at a rate of 1.68ml/min for 90 minutes. After conducting steps B, C, and D, the materialwas found to consist of nano-platelets approximately 370 nm wide×130 nmthick. FIG. 9 shows an FE-SEM image of the silver platelets obtained inthis example.

Example 10 90% Ag 10% Pd Platelets

Using the procedure as outlined in Example 1 except only one metallicprecursor solution was prepared: the metallic precursor solution wasprepared by dissolving 1.765 g AgNO₃ in 10.6 ml H₂O and then adding1.384 g Pd(NO₃)₂ solution (9.0% Pd metal basis) to provide aconcentration of 0.90 M Ag and 1.02 M Pd. The metal solution was addedat a rate of 1.68 ml/min for 7 minutes. After conducting steps B, C, andD, the material was found to consist of nano-platelets approximately 110nm wide×35 nm thick. FIG. 10 shows an FE-SEM image of the silverplatelets obtained in this example.

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be appreciated by oneskilled in the art, from a reading of the disclosure, that variouschanges in form and detail can be made without departing from the truescope of the invention as set forth in the appended claims.

1. A metallic nano-platelet comprising between 90% and 99.99% silver;and between 0.01% and 10% palladium.
 2. The metallic nano-platelet ofclaim 1, wherein the metallic nano-platelet comprises 99.1% or moresilver and 0.8% or less palladium.
 3. The metallic nano-platelet ofclaim 1, wherein the metallic nano-platelet comprises about 0.2%palladium.
 4. The metallic nano-platelet of claim 1, wherein themetallic nano-platelet comprises at least about 0.05% palladium.
 5. Themetallic nano-platelet of claim 1, wherein the degree of crystallinityis at least 0.25.
 6. The metallic nano-platelet of claim 1, wherein thedegree of crystallinity is at least 0.5.
 7. The metallic nano-plateletof claim 1, wherein the degree of crystallinity is about
 1. 8. Themetallic nano-platelet of claim 1, wherein the thickness of the metallicnano-platelet is from about 20 nm to about 300 nm.
 9. The metallicnano-platelet of claim 1, wherein the width of the metallicnano-platelet is from about 100 nm to about 1000 nm.
 10. The metallicnano-platelet of any one of claim 1, wherein the metallic nano-platelethas an aspect ratio between about 2 to about
 20. 11. The metallicnano-platelet of any one of claim 1, wherein the metallic nano-platelethas an aspect ratio of at least about
 5. 12. A plurality of the silvernano-platelets according to any one of claim 1, wherein at least 90% ofthe silver platelets have a width within the range of W±20% W, where Wis the mean width of the silver platelets.
 13. The plurality of silvernano-platelets of claim 12, wherein at least 80% of the silver plateletshave a thickness within the range of T±20% T, where T is the meanthickness of the silver platelets.
 14. A method for making silvernano-platelets, comprising the essentially simultaneous addition of asilver ion solution and a reducing solution to an acidic solution, underconditions that permit the reduction of the silver ions to metallicsilver, wherein (a) the silver ion solution comprises a plurality ofsilver ions; (b) the reducing solution comprises one or more reducingagents; (c) the acidic solution comprises one or more stabilizingagents; and (d) at least part of the reduction takes place in thepresence of palladium ions.
 15. The method of claim 14, wherein thereducing solution comprises ascorbic acid, isoascorbic acid, or a saltthereof.
 16. The method of claim 14, wherein at least a portion of thesilver ion solution further comprises palladium ions.
 17. The method ofclaim 14, wherein the silver ion solution is divided into a precedingportion and a succeeding portion, and wherein the preceding portion ofthe silver ion solution comprises palladium ions.
 18. The method ofclaim 14, wherein the total amount of palladium, relative to the totalamount of silver, is about 0.2% by weight.
 19. The method of claim 14,wherein the total amount of palladium, relative to the total amount ofsilver, is at least 0.05% by weight.
 20. The method of any one of claim14 wherein the acidic solution comprises gum arabic.
 21. A silvernano-platelet obtained in accordance with the method of any one of claim14.
 22. A silver nano-platelet obtained in accordance with the method ofclaim
 20. 23. A plurality of the silver nano-platelets according to anyone of claim 14, wherein at least 90% of the silver platelets have awidth within the range of W±20% W, where W is the mean width of thesilver platelets.
 24. The plurality of silver nano-platelets of claim23, wherein at least 80% of the silver platelets have a thickness withinthe range of T±20% T, where T is the mean thickness of the silverplatelets.